Physiological Assessment of Acute Lung Injury in the Intensive Care Unit

Abstract

The term adult respiratory distress syndrome (ARDS) defines the condition of acute respiratory failure that occurs secondary to high permeability pulmonary edema. The full-blown syndrome may complicate a wide range of both direct and indirect insults to the lung, but abnormalities of pulmonary vascular permeability are detectable in patients only at risk of developing ARDS [1]. It therefore seems likely that a spectrum of lung injury exists in such patients, with ARDS representing the most severe form. This concept has been extended recently by Murray et al. [2] who have attempted to score lung injury in terms of oxygenation deficit, chest radiography, the level of positive end-expiratory pressure (PEEP) required to oxygenate the patient, and total thoracic compliance. A value greater than 0.1 suggests a degree of lung injury, with a diagnosis of ARDS requiring a score of more than 2.5 (Table 1). Despite advances in definition and diagnosis, the therapy of ARDS remains almost entirely supportive and mortality remains around 60% [3]. Nevertheless, the increased awareness of the mechanisms involved in the pathogenesis of lung injury has led to the development of a number of therapeutic interventions including immunotherapy (e.g. endotoxin antibody, TNF antibody) and surfactant replacement. Accurate, reproducible and portable methods of detecting and quantifying pulmonary physiology are now required in order that the efficacy of these therapies can be assessed. In this chapter, some of the methods of quantifying the changes in pulmonary physiology that characterize ARDS will be reviewed.